1. Field of the Invention
The present invention relates to a diesel engine start-up assisting device that starts up a diesel engine by supplying an electric load, which will be referred to as “load” hereinafter, with current to heat up the load.
2. Description of the Related Art
In the diesel engine start-up assisting device, conventionally, there are two types of devices: glow system and intake heater system. Generally speaking, the glow system is used in a pre-combustion chamber type diesel engine, while the intake heater system is used in a direct-injection type diesel engine having a large engine displacement. The glow system has a feature of superior flash heating capability and therefore, the same system is adopted for passenger automobiles etc. universally.
In a conventional glow system, for example, there is a relay system shown in FIG. 1. In this relay system, as shown in the figure, an engine control unit (ECU) 107 detects a water temperature in a not-shown engine system. Then, in response to the detected water temperature, the ECU 107 allows a battery 101 to apply electrical current to a glow plug 105 through a fuse 102, a switch 103 and a relay 104 in order to heat up the interior of a cylinder 106 of the diesel engine, thereby improving the start-up performance of the engine, particularly in winter.
In the relay system, however, it is impossible to perform system control through on/off switching actions due to the presence of various problems about the durability of relay contacts, their operational noise and the responsibility, and therefore it is difficult to adjust the temperature inside the cylinder of the diesel engine appropriately. Furthermore, since the relay system is provided with mechanical contacts, it has a problem of failure occurrence due to life duration of relays.
Meanwhile, there is known a load drive circuit which utilizes semiconductor switching elements as electrical components to drive injectors as fuel injection devices (see Japanese Patent No. 3596415). Thus, with the adoption of this load drive circuit in the glow system, it is possible to solve the above-mentioned problem.
In the field of load drive circuits, there is a load drive circuit having a function of detecting an overcurrent of the power distribution to the load. For instance, there is employed a current mirror circuit comprising a first switching element composed of an n-channel type power MOSFET (metal-oxide semiconductor field-effect transistors) etc. which energizes or cuts off the power distribution to a load and a second switching element composed of an n-channel type power MOSFET etc. which allows a passage of current smaller than that of the first switching element, in order to detect an overcurrent.
In the current mirror circuit, it is noted that the first switching element and the second switching element are driven by the same drive command signal. Furthermore, the second switching element is constructed so as to permit the passage of current proportional to the first switching element (e.g. 1st switching element: 2nd switching element=10000: 1).
In the above-constructed load drive circuit having the function of detecting overcurrent, for example, if an abnormality, such as grounded short-circuit, occurs in a load thereby causing the passage of overcurrent through the first switching element, then a current proportional to the overcurrent flows through the second switching element. Therefore, the load drive circuit is provided with an overcurrent protective unit that detects an overcurrent flowing through the first switching element when the current flowing through the second switching element exceeds a current value corresponding to the overcurrent of the first switching element, and that inactivates the first switching element in view of protecting it.
In general, the conventional diesel engine start-up assisting device includes a plurality of loads, a plurality of first switching elements and a plurality of second switching elements, both of which correspond to the plurality of loads, and a plurality of protective units that protect the plurality of first switching elements. With this constitution, the diesel engine start-up assisting device is constructed so that, when power is distributed to at least one of the multiple loads, heat generation of the at least one load distributed with power allows a diesel engine to be started up.
Assume here that a combination of the first switching element with the second switching element is defined as a power element. In case that a plurality of power elements comprising a plurality of first switching elements and a plurality of second switching elements in combination are housed into one package IC (integrated circuit), built-in integrated circuits (ICs) in the power elements generate heat with power consumption. As an integrated circuit (IC) composed of semiconductors is hard to be normally operated in high-temperature environment, an allowable power consumption (amount) in consideration of the heat generation of IC is predetermined so as to prevent a false operation of IC caused by the heat generation with power consumption. In general, the allowable power consumption (amount) is determined on a basis of the thermal resistance of an integrated circuit (IC). Note that “thermal resistance” represents the percentage of a temperature elevation to the power consumption.
In such an integrated circuit whose thermal resistance is small, there has been attempted to unify frames for mounting multiple pairs of built-in power elements thereon for the purpose of increasing the allowable power consumption (amount). With this unification of frame, it is expected to enlarge a heat radiation area and reduce the thermal resistance as possible.